The stereochemical course of polyketide antibiotic biosynthesis is to be examined by determination of the biochemical origin, regiochemistry, and stereospecificity of hydrogen introduction/elimination during the biosynthesis of erythromycin, 6-methyl-salicyclic acid, and brefeldin A, all microbial macrolide or aromatic antibiotics. The stereochemistry of acetate-polymalonate and propionate-polymethylmalonate condensations and the overall sterospecificity of polyketide biosynthesis is to be established through analysis of the incorporation of chirally labeled acetate and propionate into these antibiotics. The experimental approach will use chiral precursors containing isotopic hydrogen labels whose incorporation into the antibiotics will be achieved in vitro and in vivo. Chemical and biochemical methods will be used for determination of labeling sites and the associated sterospecificity within the antibiotics. The results will define the biochemical relationship between fatty acid and polyketide biosynthesis, provide new information about the enzymology of polyketide biosynthesis, and examine the relationship between prostaglandin biosynthesis and the ring cyclization mechanism of other monocylopentanoid polyketide metabolites, specifically brefeldin A.